Intel on mic pre low-cut circuits!

[djmiggymigz]

Hoping that someone can either point me in the right direction via an existing post or help me out here. Currently working on a 312 preamp clone and I'm hoping/wondering if it'd be possible to include a switchable (on/off) low cut filter prior to the opamp in the circuit. I have an audient 8pre with lowcuts on every channel and I love being able to clean up a bit while tracking. Currently laying out the design in KiCad and the 312 circuit is so simple that I have a ton of empty space on the pcb and would love to implement this.

Definitely worth noting that I am certainly a beginner when it comes to circuit design. I'm thinking that the filter would be added before the opamp but after the input transformer? That's more or less just a guess though.

 

[pasarski]

Definitely. If your fine with a 1st order high pass filter you only need three components, including the switch (or two if the preamp already has some kind of a high pass filter at the input). Search "RC filter".

 

[djmiggymigz]

Definitely. If your fine with a 1st order high pass filter you only need three components, including the switch (or two if the preamp already has some kind of a high pass filter at the input). Search "RC filter".

Hmm okay so it looks like that circuit would live directly prior to the OpAmp stage right?

 

[My3gger]

Yes. One example of what you need is here:
https://rockscience.net/capi-vp312-preamp-build-and-mod/

 

[djmiggymigz]

Yes. One example of what you need is here:
https://rockscience.net/capi-vp312-preamp-build-and-mod/

wowwww this is so helpful thank you thank you!!

it's really as simple as inserting a couple of caps into the mix to roll off low frequencies??

 

[My3gger]

You are welcome.

 

[djmiggymigz]

hm so that being the case, if I followed the idea of inserting the caps inbetween the load resistor and the op-amp, I could also probably b uild out a circuit of different caps that are selectable with a rotary switch for different rolloffs yeah? Not sure if the rolloff selection would be worth the work/cost of the rotary switch but, just an idea

 

[djmiggymigz]

I suppose what rockscience did is probably best and easiest tho. just two caps on a switch for 2 different rolloff options. Probably easiest too.

Also, just realized what I think is an error in my design! KiCad clocked the R3 and In+ pin of the 2520 as grounded thus there was no connection between them.

Edit: Went thru and found several errors! Inadvertent success,, thank you!